When a bone is fractured, either by deliberately cutting it (osteotomy) or by trauma, it heals better if the bone fragments are pressed firmly together. Compression of the fragments increases the contact area across the fracture and increases stability of the bone at the fracture. It also decreases stress on any orthopedic implant.
Internal fixation of a bone fracture using bone screws is now common practice. The screw is applied across the fracture, preferably at nearly a right angle to the fracture, although the nature of the bone and of the fracture frequently dictates other angles. The distal end of the screw crosses the fracture, and when the head of the bone screw engages the proximal fragment, further rotation of the screw draws the distal fragment of the bone against the proximal fragment. Any screw that is used to achieve interfragmental compression is termed a lag screw. The two most common types of lag screws are cortical and cancellous screws. Cortical screws have fine threads on their shaft and are designed to anchor in cortical bone. Cancellous screws tend to have coarser threads and are designed to anchor in the softer cancellous bone.
Both types of lag screw generally include a threaded distal end and a proximal head. Although the screw may be threaded nearly to the head, this design requires that the proximal bone fragment be pre-bored to permit the threads to pass smoothly through the proximal fragment. More commonly, the threads on the distal end extend only far enough to ensure a positive grip in the distal fragment but not so far as to engage the proximal fragment when the screw is applied, the shaft between the threads and the head being smooth and sized no larger than the minor diameter of the threads (the maximum diameter of the thread groove). The distal side of the head, facing the shaft, is usually symmetrically convex, preferably hemispherical, and the proximal face of the proximal bone fragment is frequently lightly countersunk, in order to spread stresses in the screw and the bone most efficiently, to reduce the risk of creating a stress fracture, and to minimize the protrusion of the screw head from the face of the bone. The upper, proximal, side of the screw head is generally flat or gently rounded to permit the head to lie as close to level with the proximal bone surface as possible. The head is provided with a slot, spaced holes, a hexagonal socket, or other depression to accept the blade or tip of a drive tool or screwdriver designed to be inserted into it.
In order to eliminate the need for pre-drilling bone and tapping the distal bone fragment, bone screws are now frequently made to be self-tapping. To aid further in the placement of bone screws, the screws are frequently cannulated, having a hollow shaft and head. Cannulated screws may be placed more precisely than non-cannulated screws. The surgeon first drills a small Kirschner wire (K-wire) across the fracture, generally under fluoroscopic control. The wire may sometimes be inserted through the skin without the need of an incision. If necessary, the K-wire can be withdrawn and replaced with minimal trauma to the bone in order to place it in optimal position across the fracture. A small incision may then be made through the skin to enable the surgeon to minimize tissue trauma while placing the bone screw and to permit countersinking the bone around the point of insertion of the screw. The cannulated screw is then placed over the wire and slid down to the bone surface. A special cannulated driving tool then allows the screw to be driven into the bone along the shaft of the K-wire. The K-wire is then withdrawn and the wound over the screw is closed.
The construction and use of bone screws has become standardized to a great extent. There are of course, many variations on the details of the construction of bone screws, including for example, the use of a break-away driven element as shown in Patterson et al, U.S. Pat. No. 8,221,478.
Bone screws may also be designed merely as an anchor for attaching an external stabilizing device. Pedicle screws, such as illustrated in Mazda et al., U.S. published application US 2004/0116932 A1, are examples of such external fixation screws. The present invention is not principally concerned with such screws, although some aspects of the invention may be applicable to them.
Internal fixation bone screws may be left in the body after implantation. However, surgeons are increasingly removing fixation for a number of justifiable reasons. Irritation/inflammation, allergic reaction, and infection are common reasons to remove hardware at appropriate times. Although rare, implant rejection may occur. Furthermore, the long-term deleterious effects of a metal such as stainless steel or a titanium alloy implanted in the body are not fully understood. It is not uncommon for barometric pressure and changes in ambient temperature to cause rheumatic or osteoarthritic flare-ups. The possibility of a causal relationship influenced by hardware left in and around these areas exists. Finally, because of growing concerns over electro-magnetic radiation caused by cell phone use and other exposure, it is ideal that conductive metals be removed from the body if possible. MRI and other present electromagnetic technologies are influenced by conductive, metallic implants. Future technologies may depend on the body being free of conductive elements. Therefore, particularly when a deleterious effect is noted, a bone screw is sometimes removed, thereby allowing bone regeneration in the volume formerly occupied by the bone screw. Such removal, however, requires considerable effort and risk, as suggested by patents such as Bonati et al., U.S. Pat. No. 7,090,680, Steffee, U.S. Pat. No. 4,854,311, Vasta et al., U.S. Pat. No. 7,582,093, or Lindemann et al., U.S. published application US 2007/0270880 A1. Special screw removal kits including multiple instruments are commercially available.
When faced with having to remove a screw of the prior art, a surgeon must deal with creeping fibrosis, meaning soft tissues that creep into the screw threads making it difficult to access the screw without an incision into periosteal structures and more trauma. Fibrosis, in a worst-case scenario will make it necessary for a full screw extraction set to be utilized thereby completely bypassing the conventional methodologies of placing a screwdriver into a head. This usually requires cutting the screw or severe countersinking.
In order to overcome these problems, some bone screws are made of absorbable materials. These screws, however, are not as strong as metal screws, require drilling and tapping with metal instruments, and are transparent to x-rays. Hybrid metal and polymer screws are disclosed in Fischer et al., U.S. Pat. No. 4,711,232 and in TenHuisen et al., U.S. Pat. No. 6,916,321, but these screws add complexity and do not solve all of the problems with leaving metal in the body. Another solution has been the use of screws made of compatible bone, as in Reed, U.S. Pat. No. 5,968,047. This approach is costly and has not been entirely satisfactory.